This invention relates generally to semiconductor technology, and more particularly to a method of forming lattice matched single crystal wide bandgap II-VI compound semiconductor films over silicon.
This invention was co-funded by the Center for Space Power, a NASA-funded Center for the Commercial Development of Space and a research division of the Texas Engineering Experiment Station, a state agency and a component of The Texas AandM University System, and the U.S. Government may have certain rights in the invention.
In the field of semiconductor technology, devices have recently become available which take advantage of the different bandgap widths of various materials. For example, nanoelectronic, quantum-effect devices can take advantage of materials having different bandgap widths to create regions of varying potential energy, to allow for realization of electron tunnelling and resonance. Other applications that make use of these different bandgap widths include blue-UV (ultraviolet) light emitting devices and high efficiency solar cells.
Much of the work in these nanoelectronic and optoelectronic devices has been based on xe2x80x9cIII-Vxe2x80x9d materials, such as gallium arsenide and indium phosphide. However, these technologies have yet to become pervasive, because they are not compatible with conventional technology and systems, which are silicon based. Thus, efforts have been underway to develop silicon compatible technologies, which would allow, for example, combining conventional devices with nanoelectronic or optoelectronic devices all on the same silicon substrate.
Several II-VI compounds present materials with bandgaps greater than that of silicon and which have lattice constants compatible with that of silicon. Thus, they are attractive as materials for forming heterostructures of the kind which would allow for silicon based implementation of devices that take advantage of different bandgap width materials. Examples of such materials include ZnS, ZnSSe, ZnSTe, CaZnS, MgZnS, MnZnS, BeMgS, and BeSeTe. The ratios of the group II or VI elements in the tertiary compounds are adjusted to match the lattice constants to that of silicon.
However, difficulties in depositing these materials at a high quality (needed for such applications) on silicon have arisen because of the chemical interaction between the lattice matched layer and the silicon. For example, sulfur in such lattice matched layers reacts to form a silicon sulfide. This silicon sulfide (which is either amorphous or polycrystalline) prevents growth of high quality single crystal materials on the silicon.
Existing techniques for growing single crystal films on silicon substrates have significant disadvantages. For example, techniques for direct molecular beam epitaxy (xe2x80x9cMBExe2x80x9d) growth on thermally cleaned silicon surfaces result in poor crystal quality and rough surface morphology. Furthermore, a high density of twin defects is often present in the layers grown with these techniques. Furthermore, while twin-free single crystal layers with smooth surface morphology at particular conditions have been reported with these techniques, these conditions could not be reproduced by the present inventors for layer thicknesses above 2000 angstroms. See, xe2x80x9cGrowth of Crystalline Zinc Sulfide Films on (111)-oriented Silicon by Molecular-Beam Epitaxyxe2x80x9d, Meiso Yokoyama and Shin-ichi Ohta, J. Appl. Phys. 59, 3919 (1986); xe2x80x9cSubstrate Temperature Effect on Crystallographic Quality and Surface Morphology of Zinc Sulfide Films on (100)-oriented Silicon Substrates by Molecular-Beam Epitaxyxe2x80x9d, Meiso Yokoyama, Io-ichi Kahsior, and Shin-ichi Ohta, J. Appl. Phys. 60, 3508 (1986); and xe2x80x9cMolecular Beam Epitaxial Growth of ZnS on a (100)-oriented Si Substratexe2x80x9d, Meiso Yokoyama, Io-ichi Kashior, and Shin-ichi Ohta, J. Crystal Growth 81 73 (1987).
Another technique, which involves direct MBE growth on cleaved Si(111) surfaces, has been described by Maierhofer et al., in xe2x80x9cValence Band Offset in ZnS Layers on Si (111) grown by Molecular Beam Epitaxyxe2x80x9d, J. Vac. Sci. Technol. B 9, 2238 (1991). With this technique, however, it is extremely difficult to obtain a large area material, and it is therefore not suitable for growing such crystals for device applications.
A third technique, using chemical vapor deposition, was described by Kirabayashi et al. in xe2x80x9cEpitaxial Growth of ZnS by Metal Organic Chemical Vapor Depositionxe2x80x9d, Japan J. Appl. Phys. 24, 1590 (1985). This technique has significant disadvantages that are particularly apparent in the fabrication of heterostructures for quantum well structures. In particular, it presents difficulties in forming abrupt interfaces and precisely controlling layer thicknesses.
Therefore, a need has arisen for a method of forming a lattice matched single crystal wide bandgap compound over a silicon substrate that is suitable for formation of heterostructures. In accordance with the teachings of the present invention, a method of forming such a layer over a surface of a silicon substrate is provided which substantially reduces or eliminates disadvantages and problems associated with the prior approaches. In particular, a method is provided in which the silicon substrate is first cleaned. A passivation layer is then formed on the silicon substrate, and the lattice matched layer is formed on the passivation layer. In particular embodiments, the passivation layer may comprise arsenic, germanium, or CaF2. Furthermore, the lattice matched layer may be a compound of group II and VI materials.
An important technical advantage of the present invention is the fact that a passivation layer is provided between a silicon substrate and a layer lattice matched to the silicon. Because of this passivation layer, a relatively wide bandgap layer can be grown and then used to form heterostructures on the silicon substrate, thereby allowing for applications such as nanoelectronics and optoelectronics to be based on and compatible with silicon technology.